The present invention relates to imaging with laser devices, and in particular to transfer-type imaging of lithographic printing plates.
In offset lithography, an image to be transferred to a recording medium is represented on a plate, mat or other printing member as a pattern of ink-accepting (oleophilic) and ink-repellent (oleophobic) surface areas. In a dry printing system, the member is simply inked and the image transferred onto a recording material; the member first makes contact with a compliant intermediate surface called a blanket cylinder which, in turn, applies the image to the paper or other recording medium. In typical sheet-fed press systems, the recording medium is pinned to an impression cylinder, which brings it into contact with the blanket cylinder.
In a wet lithographic system, the non-image areas are hydrophilic in the sense of affinity for dampening (or xe2x80x9cfountainxe2x80x9d) solution, and the necessary ink-repellency is provided by an initial application of such a solution to the plate prior to inking. The fountain solution prevents ink from adhering to the non-image areas, but does not affect the oleophilic character of the image areas.
If a press is to print in more than one color, a separate printing plate corresponding to each color is required. The plates are each mounted to a separate plate cylinder of the press, and the positions of the cylinders coordinated so that the color components printed by the different cylinders will be in register on the printed copies. Each set of cylinders associated with a particular color on a press is usually referred to as a printing station.
Because of the ready availability of laser equipment and their amenability to digital control, significant effort has been devoted to the development of laser-based imaging systems. Early examples utilized lasers to etch away material from a plate blank to form an intaglio or letterpress pattern. See, e.g., U.S. Pat. Nos. 3,506,779 and 4,347,785. This approach was later extended to production of lithographic plates, for example, by removal of a hydrophilic surface to reveal an oleophilic underlayer. See, e.g., U.S. Pat. No. 4,054,094. These systems generally require high-power lasers, which are expensive and slow.
A second approach to laser imaging involves the use of thermal-transfer materials. See, e.g., U.S. Pat. Nos. 3,945,318; 3,962,513; 3,964,389; 4,395,946, 5,156,938; 5,171,650; and 5,819,661. With these systems, a polymer sheet transparent to the radiation emitted by the laser is coated with a transferable material. During operation the transfer side of this construction is brought into contact with a receiver sheet, and the transfer material is selectively irradiated through the transparent layer. Irradiation causes the transfer material to adhere preferentially to the receiver. The transfer and receiver materials exhibit different affinities for fountain solution and/or ink, so that removal of the transparent layer together with unirradiated transfer material leaves a suitably imaged, finished printing plate. Typically, the transfer material is oleophilic and the receiver is hydrophilic.
The term xe2x80x9chydrophilicxe2x80x9d is herein used in the printing sense to connote a surface affinity for a fluid which prevents ink from adhering thereto. Such fluids include water, aqueous and non-aqueous dampening liquids, and the non-ink phase of single-fluid ink systems. Thus, a hydrophilic surface in accordance herewith exhibits preferential affinity for any of these materials relative to oil-based materials. The term xe2x80x9cliquid to which ink will not adherexe2x80x9d connotes not only the traditional dampening solutions as described above, but also extends to polar fluids that may be incorporated within an ink composition itself. For example, so-called xe2x80x9cwaterbornexe2x80x9d inks (or other single-fluid ink systems) contain an aqueous fraction that will remove an inorganic protective layer in accordance herewith as the plate is used for printing.
The most common hydrophilic surface used in lithographic applications is textured metal, e.g., aluminum that has been surface-treated (typically by graining and/or anodization). Although chromium and stabilized aluminum grain surfaces exhibit good durability characteristics during printing, their hydrophilic character also renders them hygroscopic. Excessive sorption of moisture facilitates ongoing chemical reaction that may result in reduction or elimination of hydrophilic character. For this reason, if plates having such surfaces are to be stored, they typically first receive a coating of a protective, water-soluble polymer in a process known as xe2x80x9cgumming.xe2x80x9d
While suitable for many wet-printing applications, in which the protective coating is washed away during the print make-ready process, gummed plates cannot ordinarily be used in transfer applications; the protective coating is designed to be removed and therefore cannot serve as the permanent receiver surface for oleophilic transfer material. Thus, if metal-based plates are to be used as hydrophilic receivers in transfer-type applications, traditional approaches to surface stabilization cannot generally be employed. The plate must be maintained in a highly clean and moisture-free environment to avoid image degradation.
The present invention provides metal-based printing members suitable for transfer-type imaging. In general, these printing members comprise a metal substrate and a hydrophilic, polymeric coating thereover. Desirably, the polymeric coating is crosslinked and withstands repeated application of fountain solution during printing. The polymeric coating can, however, undergo degradation where exposed to fountain solution so long as the ink-receptive portionsxe2x80x94the areas where oleophilic material has been transferred onto the hydrophilic coatingxe2x80x94remain intact. This is because the coating acts to stabilize the underlying textured metal surface, so that even if the coating is worn completely away, a suitably hydrophilic metal surface will be exposed.
Accordingly, in a first aspect, the invention relates to a transfer-type method of imaging a recording construction. In this aspect, the invention utilizes a donor member comprising a transferable oleophilic material and a receiver member comprising a metal substrate having a hydrophilic, polymeric layer thereover. The donor member is irradiated in a pattern corresponding to an image so as to cause displacement of the oleophilic material onto the hydrophilic, polymeric layer of the receiver member. The result is a lithographic imagexe2x80x94i.e., an imagewise pattern of ink-accepting and hydrophilic regions.
In a second aspect, the invention relates to an ink-jet (or similar jetting) method of imaging a recording construction. In this aspect, the invention utilizes a receiver member comprising a metal substrate and a hydrophilic, polymeric layer thereover. An oleophilic material, such as liquid inks based on hydrocarbon solvents, is ejected onto the polymeric layer in an imagewise pattern, thereby creating a lithographic image.
In a third aspect, the invention relates to a hydrophilic-surfaced lithographic printing precursor capable of receiving thereon an oleophilic material in an imagewise pattern. The precursor has a metal substrate whose surface is hydrophilic and, over the surface, a hydrophilic, polymeric layer. The precursor can receive oleophilic material in an imagewise pattern by means of any suitable transfer technique, such as thermal transfer from a sheet or by jetting.